This invention relates to ultrasonic systems and, more particularly, to apparatus for imaging sections of a body by transmitting ultrasonic energy into the body and determining the characteristics of the ultrasonic energy reflected therefrom.
During the past two decades ultrasonic techniques have become more prevalent in clinical diagnosis. Such techniques have been utilized for some time in the field of obstetrics, neurology and cardiology, and are becoming increasingly important in the visualization of subcutaneous blood vessels including imaging of smaller blood vessels.
Various fundamental factors have given rise to the increased use of ultrasonic techniques. Ultrasound differs from other forms of radiation in its interaction with living systems in that it has the nature of a mechanical wave. Accordingly, information is available from its use which is of a different nature than that obtained by other methods and it is found to be complementary to other diagnostic methods, such as those employing X-rays. Also, the risk of tissue damage using ultrasound appears to be much less than the apparant risk associated with ionizing radiations such as X-rays.
The majority of diagnostic techniques using ultrasound are based on the pulse-echo method wherein pulses of ultrasonic energy are periodically generated by a suitable piezoelectric transducer such as a lead zirconate-titanate ceramic. Each short pulse of ultrasonic energy is focused to a narrow beam which is transmitted into the patient's body wherein it eventually encounters interfaces between various different structures of the body. When there is a characteristic impedance mismatch at an interface, a portion of the ultrasonic energy is reflected at the boundary back toward the transducer. After generation of the pulse, the transducer operates in a "listening" mode wherein it converts received reflected energy or "echoes" from the body back into electrical signals. The time of arrival of these echoes depends on the ranges of the interfaces encountered and the propagation velocity of the ultrasound. Also, the amplitude of the echo is indicative of the reflection properties of the interface and, accordingly, of the nature of the characteristic structures forming the interface.
There are various ways in which the information in the received echoes can be usefully presented. In one common technique, the electrical signal representative of detected echoes are amplified and applied to the vertical deflection plates of a cathode ray display. The output of a time-base generator is applied to the horizontal deflection plates. Continuous repetition of the pulse/echo process in synchronism with the time-base signals produces a continuous display, called an "A-scan", in which time is proportional to range, and deflections in the vertical direction represent the presence of interfaces. The height of these vertical deflections is representative of echo strength.
Another common form of display is the so-called "B-scan" wherein the echo information is of a form more similar to conventional television display; i.e., the received echo signals are utilized to modulate the brightness of the display at each point scanned. This type of display is found especially useful when the ultrasonic energy is scanned transverse the body so that individual "ranging" information yields individual scanlines on the display, and successive transverse positions are utilized to obtain successive scanlines on the display. This type of technique yields a cross-sectional picture in the plane of the scan, and the resultant display can be viewed directly or recorded photographically or on magnetic tape. The transverse scan of the beam may be achieved by a reflector which is scanned mechanically over a desired angle. It is generally considered desirable that the ultrasound reflector be mechanically scanned at a linear rate which is compatible with a given field rate of a television-type display. Accordingly, an acceptable scan pattern would be controlled by a sawtooth energizing waveform which rises linearly over most of its period and has a relatively short "flyback" (related to vertical blanking of the display), the flyback being as short as possible so that the operational duty cycle is as high as possible.
The described scan of the ultrasound reflector is considered advantageous from the standpoint of compatibility with television-type displays and is intended to provide advantageous linearity during the scan, but it is found that operational difficulties arise. In particular, it is known that ultrasound is highly reflected at liquid-gas interfaces and this has led to the technique of coupling the ultrasound through a fluid. When the reflective scanner is contained in a fluid medium such as water, linear movement and relatively fast flyback can be difficult to attain and/or be mechanically inefficient requiring high input power. This is especially true for equipments wherein relatively large-area scanners, with their attendant inertial resistance to acceleration and deceleration in a fluid, are utilized.
It is one of the objects of this invention to provide an imaging system which is generally responsive to the problems of the prior art, as set forth.